1. Field of the Invention
The present invention relates to a circuit for starting and operating a plurality of arc discharge lamps, and more particularly, to an electrical circuit for starting and ballasting one lamp at a time of a plurality of arc discharge lamps, the circuit including means for selecting the individual lamp to be started and operated.
2. Description of the Prior Art
Many arc discharge lamp applications require a backup lamp. Circuits are provided, therefore, wherein multiple lamps share a single power supply. With such an arrangement, the lamps are operated individually, a single lamp being selected by a selector switch. This has been accomplished traditionally by connecting the lamps in parallel across the output of the power supply and opening the circuit to the lamps not intended to be lighted by the use of a relay contact connected serially with each lamp. This approach, although apparently simple, has several problems. They include these. (1) The relay contacts cannot be opened when the lamp is lighted because a generated DC arc is capable of melting the contacts before the arc breaks and is quenched. This problem is usually solved by adding a circuit which removes the input power to the power supply thus reducing output to zero before switching to the next lamp to be operated. (2) The open relay contacts associated with the unselected lamps are subjected to a high voltage stress level of 10 to 30 KV during lamp starting. This mandates the use of high-voltage power relays, and such relays are very expensive indeed. (3) Circuits required to operate these high-voltage power relays provide the "power down" function during switching and these high-voltage power relays themselves are often not capable of operating a sufficient number of "lamp switching cycles" to meet the needs of the application. In short, they are unreliable.
Furthermore, most prior art power supplies for operating arc discharge lamps have been of the "non-electronic" type; i.e., they are of the inductor, rectifier filter type. Such a circuit, having no inherent low-level electronic means to control the output as regards the "on" or "off" state, does not lend itself to a simple means of transferring the power supply output from one lamp to another.
It is desirable, therefore, to provide an electrical circuit for starting and ballasting a plurality of arc discharge lamps, the circuit being provided with means for transferring the output of the power supply from one lamp to another, thereby to effect selective individual operation of the lamps.
Accordingly, it is an object of the present invention to provide a circuit for the selective individual operation of multiple arc discharge lamps, the circuit including means capable of low-voltage operation while transferring circuit output from one lamp to another.